Short answers, pulled from the story.
Street Fighter X Tekken is a 2012 video game developed by Capcom that combines characters from the Street Fighter and Tekken franchises. The PlayStation 3 and Xbox 360 versions became available starting on the 31st of July 2012 while PC users gained access later on the 6th of February 2013.
The Cross Gauge system allows players to build power over time and unleash techniques like EX Attacks and Super Arts when the meter fills up. When the gauge reaches full capacity players can chain together each character's Super Combo into a single straight attack known as Cross Arts or use Cross Assault to control both team members simultaneously until the gauge depletes completely.
The base roster includes thirty-eight playable characters split evenly between Street Fighter and Tekken franchises with additional downloadable content adding twelve more fighters six from each side. Five exclusive playable characters appeared only on Sony platforms including Cole MacGrath from Sucker Punch's Infamous series alongside mascots Toro Inoue Kuro Bandai Namco's Pac-Man and Capcom's Mega Man.
A mysterious cubical object crash-landed somewhere deep within Antarctica releasing water-like energy that amplified combatants' abilities whenever beings came into conflict near it. This phenomenon named Pandora allowed characters dropped below 25% health remaining to be sacrificed to activate Pandora Mode which granted the surviving fighter increased strength plus an infinite Cross Gauge but carried a strict time limit.
Within two months after release Street Fighter X Tekken sold over one million copies globally representing approximately forty percent below Capcom's expectations of two million units sold. By the 31st of March 2014 total sales reached 1.7 million rising slightly from 1.4 million recorded by the 31st of March 2012 due partly to cannibalization within fighting game genre occurring too quickly.